1. Field of the Invention
The present invention relates to a four-cycle engine for driving a work machine such as a trimmer, a grass cutter or a rammer and, in particular, an improvement in a breather structure in a four-cycle engine for a work machine wherein a crankcase of an engine body rotatably supports a crankshaft linked to the work machine, lubricating oil is stored in a crank chamber formed inside the crankcase, and an intake system is connected to a cylinder head of the engine body, the axis of a cylinder bore of the engine body being almost vertical when the work machine is being used.
2. Description of the Related Art
The attitude of a work machine such as a trimmer, a grass cutter or a rammer varies from when it is operating to when it is not operating. The oil surface inside the crank chamber also varies from when it is operating to when it is not operating. It is therefore necessary to arrange the breather structure for guiding breather gas from the crank chamber into an intake system so that the lubricating oil is prevented from entering the intake system when it is not operating. A breather structure arranged from the above-mentioned point of view is disclosed in, for example, Japanese Patent Publication No. 1-51647.
The above-mentioned Japanese Patent Publication No. 1-51647 discloses a breather structure that prevents the lubricating oil from entering the intake system even when the engine body is turned over through 360 degrees. In this breather structure, a mist separator that is separate from the engine body is connected to the upper part of a cylinder head via a first check valve that closes when the engine body turns over and to the lower part of a cylinder block via a second check valve that closes when the engine body turns over. Since the mist separator is placed at a position away from the engine body, the overall dimensions of the engine increase. Moreover, in an engine for a work machine such as a grass cutter or a rammer, the engine body may be tilted thereby making the cylinder bore axis almost horizontal when the machine is not being used. It is therefore necessary to prevent the lubricating oil from entering the intake system under conditions where the engine body is tilted downward. However, the technique disclosed in the above-mentioned publication cannot deal with such a state where the engine body is tilted downward.
The present invention has been carried out in view of the above-mentioned circumstances, and it is an object of the present invention to provide a breather structure in a four-cycle engine for a work machine that can prevent the lubricating oil from entering the intake system when the engine body is tilted downward while avoiding any increase in the overall engine dimensions.
In order to achieve the above-mentioned object, in accordance with a first aspect of the present invention, there is proposed a breather structure in a four-cycle engine for a work machine in which a crankcase of the engine body rotatably supports a crankshaft linked to the work machine, lubricating oil is stored in a crank chamber formed inside the crankcase, and an intake system is connected to a cylinder head of the engine body, an axis of a cylinder bore of the engine body being almost vertical when the work machine is being used, wherein the breather structure includes: a first breather chamber; a first through passage for providing communication between the first breather chamber and the crank chamber; a second breather chamber placed in the vicinity of the intake system on the side approximately opposite the first breather chamber relative to the axis of the cylinder bore; a second through passage for providing communication between the second breather chamber and the crank chamber; and a communicating passage for providing communication between the first and second breather chambers, the first breather chamber, the first through passage, the second breather chamber, the second through passage and the communicating passage being provided in the engine body so that the first and second through passages communicate with the lower parts of the first and second breather chambers that are positioned above the oil surface inside the crank chamber when the work machine is being used and the communicating passage opens into the upper part of the second breather chamber. A guide pipe that communicates with the upper part of the second breather chamber when the work machine is being used is connected to the intake system, the second through passage is formed such that an open end of the second through passage inside the crank chamber is positioned above the oil surface inside the crank chamber regardless of the attitude of the engine body when the engine body is tilted downward such that the axis of the cylinder bore becomes almost horizontal, and the route from the first through passage to the communicating passage via the first breather chamber being made in a shape that prevents the lubricating oil inside the crank chamber from entering the communicating passage when the engine body is tilted downward such that the communicating passage is positioned beneath the axis of the cylinder bore.
In accordance with the above-mentioned arrangement, when the work machine is being used, breather gas that is generated inside the crank chamber is guided to the intake system via the first through passage, the first breather chamber, the communicating passage, the second breather chamber and the guide pipe and is guided to the intake system via the second through passage, the second breather chamber and the guide pipe. The lubricating oil separated from the breather gas inside the first and second breather chambers is returned to the crank chamber through the first and second through passages. Because the second through passage is formed such that its open end inside the crank chamber is positioned above the oil surface inside the crank chamber regardless of the attitude of the engine body when the engine body is tilted downward such that the axis of the cylinder bore becomes almost horizontal when the work machine is not being used, it is possible to prevent the lubricating oil inside the crank chamber from entering the second breather chamber via the second through passage. Moreover, the route from the first through passage to the communicating passage via the first breather chamber is shaped so as to prevent the lubricating oil inside the crank chamber from entering the communicating passage even when the engine body is tilted downward so that the communicating passage is positioned beneath the axis of the cylinder bore. The lubricating oil inside the crank chamber therefore does not enter the second breather chamber from the first through passage via the first breather chamber and the communicating passage. In this way, the lubricating oil inside the crank chamber can be prevented from entering the second breather chamber regardless of the attitude of the engine body when it is tilted downward so that the axis of the cylinder bore becomes almost horizontal, and the lubricating oil can reliably be prevented from entering the intake system. Furthermore, because the first and second breather chambers are provided in the engine body, the overall dimensions of the engine do not increase when using the above-mentioned arrangement for preventing the lubricating oil from entering the intake system.
Furthermore, in accordance with a second aspect of the present invention, in addition to the above-mentioned first aspect, the second through passage is formed from a passage hole that is directly provided in the engine body so as to communicate with the second breather chamber and a pipe that is secured to the engine body so as to communicate with the passage hole.
In accordance with the above-mentioned arrangement, the second through passage, which has a complex shape so that its open end is positioned above the oil surface inside the crank chamber regardless of the attitude of the engine body when the engine body is tilted over such that the axis of the cylinder bore becomes almost horizontal, can be formed by a simple arrangement.
The above-mentioned objects, other objects, characteristics and advantages of the present invention will become apparent from an explanation of a preferable embodiment that will be described in detail below by reference to the attached drawings